With the explosive spread of data communication networks such as the Internet, needs for higher capacities of optical communication networks are being increasingly growing. To address the growing network needs, wavelength division multiplexing communication has been put to practical use. In recent years, there have also been growing needs for wavelength selective switches (WSSs) that enable channel switching at each wavelength. A conventional wavelength selective switch is disclosed in PTL 1.
FIG. 10 a diagram depicting an example of the wavelength selective switch disclosed in PTL 1. In FIG. 10, a wavelength division multiplexing signal received through one of input and output optical fibers 1 to 10 propagates along a forward optical path 28 shown by a solid line while being diverged. The signal is converted into parallel beams by a concave mirror 12, and the parallel beams travel along a forward optical path 27 and enter a diffraction grating 14. The wavelength division multiplexing signal having entered the diffraction grating 14 is angularly diverged by the diffraction grating 14 and thus diffracted in different directions depending on the wavelength. The signal thus propagates in the direction of an optical path 23 shown by a solid line. The propagated optical signal is converted by a cylinder lens 13 into a condensed beam in a direction perpendicular to the sheet of FIG. 10. The condensed beam enters the convex mirror 12. At this time, the beam enters the convex mirror 12 as parallel beams in a wavelength demultiplexing direction (in a horizontal direction on the sheet of FIG. 10) and as a convergent beam in a switch axis direction (in a vertical direction in the sheet of FIG. 10). Thus, a beam waist is formed on a concave lens 13.
The optical signal reflected by the concave mirror then propagates as a dispersive beam in the vertical direction in the sheet of FIG. 10 and enters the cylinder lens 13 again. The beam is then converted by the cylinder lens 13 into parallel beams, which enter a spatial deflection element 15. On the other hand, in the wavelength demultiplexing direction (in the horizontal direction in the sheet of FIG. 10), the beams become a condensed beam, which is reflected by the concave mirror 12 and propagates directly toward the spatial deflection element 15.